How to copy paste positions of multiple vertexes (UV Mapping)
Blender shouldn't delete UV maps when joining materials.  There are a few caveats:

1) If there are more UV maps than Blender allows between the two objects, UV maps will be dropped.  I believe Blender will preserve first all UV maps of the active selection.

2) If a mesh loses its faces, it loses its UV, as Blender stores UV separate from vertices (not the only way to do it, btw), so there's no such thing as a string of edges with any UV.

When you join two objects, any UV maps with the same name will be merged.  So if you want one UV map to join with a particular other UV map, just make sure they have the same name.

Finally, there are a few tools that you can use to copy UV maps.  Assuming identical topology, ctrl-l Copy UV works.  The data transfer modifier also permits the copying of UV via Face Corner Data.  Data transfer modifier can be vertex group limited, and behaves perfectly on seamless, identical meshes, although in my experience it doesn't do a great job with seams/islands (maybe I just don't know all the tricks yet.)